tests: add sync_with_i3 after open_window

We need to set dont_map => 1 on the sync window to prevent an endless loop.

Further, t/219-ipc-window-focus.t made assumptions about windows being named
incrementally, and that assumption is broken by the sync window opened by the
first sync_with_i3 call from open_window, so use the more reliable ->name.

testcases/lib/i3test.pm.in

@@ -350,6 +350,12 @@ sub open_window {
 
     $window->map;
     wait_for_map($window);
+
+    # MapWindow is sent before i3 even starts rendering: the window is placed at
+    # temporary off-screen coordinates first, and x_push_changes() sends further
+    # X11 requests to set focus etc. Hence, we sync with i3 before continuing.
+    sync_with_i3();
+
     return $window;
 }
 
@@ -688,6 +694,7 @@ sub sync_with_i3 {
         $_sync_window = open_window(
             rect => [ -15, -15, 10, 10 ],
             override_redirect => 1,
+            dont_map => 1,
         );
     }